OK so I'm not sure if have an extended battery already and are just wondering if you got a new one, of a different mah of course, if it would work with the back that you have, or if you want a new one that is bigger than the oem and are asking if you have to replace the back, but here is the answer.

It looks like the 1900 will fit into the original back cover. Now when I had the incredible and got the 1700 or 1900 battery it fit into the cover, but it was tight. I mean like on the verge or popping the cover off tight, but as long as you have all of your little tabs and are careful you should be alright.